1. Field of the Invention
The present invention relates to a vehicle behavior control apparatus, which stabilizes the behavior of a vehicle such as an automobile while the vehicle is in motion, and more particularly to an apparatus that executes deceleration control to reduce the speed of the vehicle in order to avoid excessive rolling of the vehicle or to prevent the vehicle from overturning during steering of the vehicle.
2. Description of the Related Art
If a comparatively large centrifugal force acts on the center of gravity of a vehicle when the vehicle is steered, yaw direction behavior of the vehicle is likely to become more unstable than in a normal turn. In certain cases, the rolling behavior of the vehicle may also become unstable, for example when excessive rolling is generated in a direction an upper portion of the center of the gravity of the vehicle moves toward the outer side of the turn, leading to an increase in the possibility of the vehicle overturning. Accordingly, various deceleration control techniques and behavior control techniques for stabilizing the yaw direction behavior of the vehicle and suppressing rolling of the vehicle by reducing the speed of the vehicle during a turn in order to reduce the centrifugal force acting on the vehicle have been proposed and put to practical use.
For example, Japanese Patent Application Publication No. 2000-52963 (JP-A-2000-52963) describes a technique for directly suppressing rolling in a vehicle during a turn in which a lateral acceleration threshold (rollover threshold acceleration) for preventing the vehicle from overturning is estimated from the state of the vehicle during a turn, and a target deceleration (as well as a target yaw moment) of the vehicle is determined in consideration of the overturning rollover threshold acceleration.
In certain cases during a control to stabilize the turning behavior of a vehicle following a rapid turn-back steering in a large steering angle, it is not possible to suppress yaw/roll variation in the vehicle when a generated yaw moment varies in the vicinity of a limit value at which a wheel lateral force required to realize a target lateral acceleration corresponding to a target yaw rate (determined based on quantities of state such as the steering angle and the vehicle speed) can be realized at the frictional characteristic of the current road surface (in other words, a generated yaw moment may vary such that at certain times the target lateral force value is realizable and at other times the target lateral force value is unrealizable). In response to this problem, Japanese Patent Application Publication No. 2006-193038 (JP-A-2006-193038) describes a technique for stabilizing the yaw/roll behavior of a vehicle during a turn in which a controlled variable (a control force of a turn outer wheel) of an actuator (for example, a braking force control unit for each wheel) that generates the yaw moment is modified in accordance with a determination as to whether the target lateral acceleration can be realized at the frictional characteristic of the current road surface.
Further, Japanese Patent Application Publication No. 2000-168524 (JP-A-2006-168524) describes a vehicle behavior stabilization control for use during emergency steering in which the braking force of each wheel is controlled (a yaw moment is generated) during emergency steering so that yaw direction behavior of the vehicle shifts to a direct advancement state, for example, thereby preventing the behavior of the vehicle from becoming unstable due to excessive steering by a driver when a yaw rate control response is delayed.
In behavior control (or deceleration control) for decelerating a vehicle in order to stabilize yaw behavior and suppress excessive rolling during steering or a turn, an index value (to be referred to hereafter as an “actual turning index value”) representing the actual turning state of the vehicle, i.e. the yaw rate, lateral acceleration, and so on, is typically compared with a target value of the turning index value (in this case, a requested value corresponding to the steering of the driver, which is calculated based on quantities of variables such as the steering angle and the vehicle speed; to be referred to hereafter as a “target turning index value”), and when the magnitude of the target turning index value is greater than the magnitude of the actual turning index value, control is performed to increase the deceleration of the vehicle. When the magnitude of the target turning index value is greater than the magnitude of the actual turning index value, the centrifugal force generated by the turn request issued by the driver is excessive in light of the current speed of the vehicle, and therefore rolling that biases the upper portion of the center of gravity of the vehicle toward the outer side of the turn may increase.
Hence, in the control described above, the vehicle is decelerated in accordance with the deviation between the target turning index value and the actual turning index value, thereby reducing the vehicle speed such that the centrifugal force acting on the vehicle decreases, the yaw behavior is stabilized, and the possibility of a rollover is reduced. In other words, in this deceleration control, the vehicle behavior is stabilized by controlling the vehicle speed so that the turning state requested by the driver conforms to an actually achievable turning state.
However, if the deceleration control is executed on an actual vehicle in accordance with the deviation between the target turning index value and the actual turning index value, as described above, the deviation between the target turning index value and the actual turning index value may decrease temporarily due to a response delay in the actual steering angle of the vehicle (or the actual turning index value) relative to the steering performed by the driver when the steering speed of the driver is comparatively high, the steering angle is large, or the steering direction is switched comparatively quickly (for example, during a sharp turn, during emergency steering to change lanes in order to avoid a frontward obstacle, and so on), and as a result, the vehicle may not be possible to achieve sufficient deceleration.
As shown in FIG. 5A, for example, when a driver executes a lane-change in an actual vehicle by manipulating the steering angle such that the target turning index value varies as shown by the solid line in FIG. 5B, and this variation is rapid, the actual turning index value of the vehicle displaces at a delay relative to the target turning index value, as shown by the dotted line in the drawing. Hence, in this case, a response delay occurs in the actual turning index value when a turn-back is performed (a1 to b0 in the drawing) during a first steering operation (a0 to b0 in the drawing), which reduces the deviation between the target turning index value and the actual turning index value, and as a result, the deceleration to be applied to the vehicle decreases based on the deviation. When the vehicle speed has not decreased sufficiently at the start of a second steering operation for turning the vehicle in the opposite direction (b0 onward), the vehicle body is jolted back and greater centrifugal force than that of a normal steering operation acts on the vehicle. As a result, the yaw behavior of the vehicle is likely to become unstable and increased body roll is likely to occur.